A typical miniature x-ray tube includes an evacuated ceramic tube with a cathode structure at one end of the tube and an anode structure at or near an opposite end of the tube. Traditionally, the cathode is heated to facilitate releasing electrons, and a high-voltage electric field is established between the cathode and the anode to accelerate the released electrons toward, and possibly beyond, the anode. There may be a short focusing element within the tube. If present, the focusing element includes electrically conducting material for creating an electric field. The focusing element is formed such that the electric field tends to concentrate the flow of electrons into a compact stream. The effectiveness of any focusing depends to a significant degree on the size of the area on the cathode from which the electrons are emitted. The smaller the area, the easier it is to develop a well defined small spot on the target.
The electron beam strikes a target at the far end of the ceramic tube, resulting in the production of x-rays. The target may be the anode or another structure. The target usually includes a thin, heavy metal coating, such as gold (Au) or tungsten (W), on the surface of a material that allows the x-rays to pass through with little attenuation. In some cases, the x-ray beam may be taken off of a more conventional solid, x-ray opaque target at an angle as scattered x-rays. In either case, the x-rays are produced from a spot on the target where the electron beam strikes the target.
To improve electron emissions from cathodes, most cathodes are now made from thoriated tungsten using a process described by Langmuir. In that process, about 2% thorium oxide is mixed with tungsten. Cathodes made of this material are then “activated” by heating them to about 2800 degrees Kelvin (K), which reduces any thorium oxide to a mono layer of metallic thorium on the surface of the tungsten. Carbon is added to the surface to carbonize some of the tungsten to tungsten carbide, which limits the rate of evaporation of the thorium from the surface. The result is a cathode that has several orders of magnitude more emission than pure tungsten. Other details regarding construction of prior-art miniature x-ray tubes are disclosed in U.S. Pat. No. 7,236,568.
In many applications, it is important that the area or a dimension of the spot on the target is as small as possible. However, it has been found that a conventional x-ray tube produces a spot of x-ray emissions surrounded by an undesirable “halo” of x-ray emissions, as a result of heat spreading in the cathode, as described in the following paragraphs.
Traditionally, the cathode is either a directly heated filamentary cathode or a planar cathode. U.S. Pat. No. 6,320,932 discloses heating a cathode by a laser light source. The use of a laser heat source makes planar cathodes easier to implement. In addition, heating a small area in the center of a thin metal cathode gives a more intense emission from the heated area than from unheated areas. An electron beam spot on the order of a few hundred microns in diameter is achievable using a laser-heated planar cathode.
However, heat is conducted from the central, directly heated area of the cathode to adjacent portions of the cathode, which causes the cooler portion of the cathode to emit electrons, albeit at a much lower rate, such as about 1/20 to about 1/100 that of the central, directly heated part of the activated cathode area. This results in a “halo” around the x-ray spot on the target. In a miniature x-ray tube, an exemplary cathode is on the order of 2-3 mm in diameter, and the central electron beam is about 0.2 mm in diameter. Thus, the area of the halo may be approximately 100 times the area of the central spot. Such a halo forms an undesirable background in a measurement.